A typical video display unit, such as an LCD display unit, comprises a display matrix capable of displaying the video signal frames of the received video signal. A typical video signal comprises a number of frames, which when displayed at appropriate speed, such as 25 frames per second, provide the viewer with an impression of viewing a continuous scene. A typical video display unit comprises a display matrix controller, which when a new frame is to be displayed, reprograms the consecutive pixels of the display matrix. The reprogramming may be made by transmitting consecutive pixel values to the matrix or by sending consecutive batches of a group of pixels, e.g. 16 pixels. For large display matrices, such as having a resolution of 1920×1080 pixels (Full HD resolution, as defined by ITU-R BT.709), the number of pixels to be reprogrammed is quite high and requires considerably long time to redraw a frame. For example, typical LCD displays need about 5 ms to redraw the currently displayed frame to display a next frame, the time called a frame redraw time. An exemplary LCD driving circuit is disclosed in the U.S. Pat. No. 7,271,793 “Liquid crystal display device, driving method for liquid crystal display devices and inspection method for liquid crystal display devices”.
A stereoscopic video signal comprises alternate left and right frames, i.e. frames to be viewed by the left eye and the right eye of the viewer, to be displayed at a higher speed than a mono video signal, such as 60 frames per second. The signal may be viewed by so-called shutter glasses, such as LCD shutter glasses, the operation of which is synchronized with the display. When the left frame is displayed, the left glass is open and the right glass is shut, while when the right frame is displayed, the left glass is shut and the right glass is open. A basic stereoscopic system of this type is disclosed in the U.S. Pat. No. 4,424,529 “Remotely triggered portable stereoscopic viewer system”.
It is important to synchronize the operation of shutter glasses such that a glass is open only when the frame is completely redrawn for a given eye, so as to provide the viewer with a complete picture where a given eye will never see and section of a frame meant for the other eye. Therefore, during redrawing of a frame, the left glass and the right glass are both shut. When the frames are displayed at a rate of 60 frames per second, the time allotted for a single frame for one eye is 16.7 ms, wherein the frame is displayed for 11.7 ms and redrawn for 5 ms. In an ideal situation, when the shutter glasses are perfectly synchronized with the display, the active viewing coefficient, defining the percentage of time at which one eye of the viewer may watch the video signal, is 11.7 ms/2*16.7 ms=35%. The coefficient can be much worse for older types of displays having longer redraw time, such as 20% for a 10 ms redraw time. In practice, there may be an additional shutters closing time necessary to be introduced in order to compensate for various signal delays. Various attempts have been made to improve the synchronization, in order to arrive at a high active viewing coefficient while keeping proper synchronization, such as the method disclosed in the U.S. Pat. No. 6,678,091 “System and method to synchronize one or more shutters with a sequence of images”.
In case of viewing stereoscopic video signals via shutter glasses, the lower the active viewing coefficient, the less information is received by the viewer's eyes and the worse impression of the viewed signal, due to worse brightness perception, since the signal is viewed only for a fraction of time as compared to a typical mono signal. The aim of the present invention is to improve the viewer impression when viewing a stereoscopic video signal. This aim is achieved by providing an improved method for controlling a display matrix and an improved display matrix controller, which reduce the frame redraw time and provide a uniform brightness perception for all frames of stereoscopic video signals.